2-stage asynchronous starting of a download

src/contentmanager.cpp

@@ -525,16 +525,36 @@ void ContentManager::downloadBook(const QString &id)
         return;
     }
 
-    startDownload(id);
+    mp_library->addBookBeingDownloaded(book, downloadPath);
+    mp_library->save();
+
+    DownloadManager::addRequest(DownloadState::START, id);
+    const auto downloadState = DownloadManager::getDownloadState(id);
+    managerModel->setDownloadState(id, downloadState);
 }
 
+// This function is called asynchronously in a worker thread processing all
+// download operations. The call is initiated in downloadBook().
 void ContentManager::startDownload(QString id)
 {
     kiwix::Book book = getRemoteOrLocalBook(id);
     const auto downloadPath = getSettingsManager()->getDownloadDir();
+    // downloadPath may be different from the value used in
+    // downloadBook(). This may happen in the following scenario:
+    //
+    // 1. aria2c is stuck because of having to save to
+    //    slow storage (and the fact that it is a single-threaded
+    //    application). This may result in startDownload() being
+    //    called with significant delay after downloadBook().
+    //
+    // 2. The user changes the download directory after starting
+    //    a download.
+    //
+    // That's why the checkThatBookCanBeDownloaded() check is repeated here.
 
     std::string downloadId;
     try {
+        DownloadManager::checkThatBookCanBeDownloaded(book, downloadPath);
         downloadId = DownloadManager::startDownload(book, downloadPath);
     } catch ( const KiwixAppError& err ) {
         emit error(err.summary(), err.details());
@@ -544,8 +564,6 @@ void ContentManager::startDownload(QString id)
     book.setDownloadId(downloadId);
     mp_library->addBookBeingDownloaded(book, downloadPath);
     mp_library->save();
-    const auto downloadState = DownloadManager::getDownloadState(id);
-    managerModel->setDownloadState(id, downloadState);
     emit(oneBookChanged(id));
 }
 

src/contentmanager.h

@@ -119,7 +119,7 @@ private: // functions
     const kiwix::Book& getRemoteOrLocalBook(const QString &id);
     QString getRemoteLibraryUrl() const;
 
-    void startDownload(QString bookId);
+    void startDownload(QString bookId) override;
     void removeDownload(QString bookId);
     void downloadDisappeared(QString bookId);
     void downloadCompleted(QString bookId, QString path);

src/downloadmanagement.cpp

@@ -131,7 +131,7 @@ void DownloadManager::processDownloadActions()
         const Request req = m_requestQueue.dequeue();
         if ( !req.bookId.isEmpty() ) {
             switch ( req.action ) {
-            case DownloadState::START:  /* startDownload(req.bookId); */ break;  // API problem
+            case DownloadState::START:  startDownload(req.bookId);  break;
             case DownloadState::PAUSE:  pauseDownload(req.bookId);  break;
             case DownloadState::RESUME: resumeDownload(req.bookId); break;
             case DownloadState::CANCEL: cancelDownload(req.bookId); break;
@@ -280,12 +280,15 @@ std::string DownloadManager::startDownload(const kiwix::Book& book, const QStrin
     } catch (std::exception& e) {
         throwDownloadUnavailableError();
     }
-    m_downloads.set(bookId, std::make_shared<DownloadState>());
     return downloadId;
 }
 
 void DownloadManager::addRequest(Action action, QString bookId)
 {
+    if ( action == DownloadState::START ) {
+        m_downloads.set(bookId, std::make_shared<DownloadState>());
+    }
+
     if ( const auto downloadState = getDownloadState(bookId) ) {
         m_requestQueue.enqueue({action, bookId});
         if ( action != DownloadState::UPDATE ) {

src/downloadmanagement.h

@@ -160,8 +160,6 @@ public: // functions
     // successful download
     void checkThatBookCanBeDownloaded(const kiwix::Book& book, const QString& downloadDirPath);
 
-    // returns the download id
-    std::string startDownload(const kiwix::Book& book, const QString& downloadDirPath);
     void removeDownload(QString bookId);
 
     DownloadStatePtr getDownloadState(QString bookId) const
@@ -175,6 +173,10 @@ signals:
     void downloadCancelled(QString bookId);
     void downloadDisappeared(QString bookId);
 
+protected:
+    // returns the download id
+    std::string startDownload(const kiwix::Book& book, const QString& downloadDirPath);
+
 private: // types
     struct Request
     {
@@ -190,6 +192,7 @@ private: // types
 
 private: // functions
     void processDownloadActions();
+    virtual void startDownload(QString bookId) = 0;
     void pauseDownload(const QString& bookId);
     void resumeDownload(const QString& bookId);
     void updateDownload(QString bookId);